Optimization of composition and heat treatment of age-hardened Pt-Al-Cr-Ni alloys
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INTRODUCTION
THE Ni-base superalloys have been used in hightemperature applications since the late 1940s. Their success is due to well-balanced properties such as creep strength and low-temperature ductility. High creep strength is achieved by finely distributed precipitates, coherently embedded in a face-centered-cubic (fcc) matrix. Coherency indicates a high stability of precipitates, i.e., very low surface energy per unit area of precipitate/matrix interface. Therefore, the driving force for particle coarsening is low. The precipitate phase in Ni-base superalloys is an L12-ordered intermetallic of the prototype Ni3Al, which shows an anomalous increase of the mechanical strength with increasing temperature at elevated temperature.[1] This very unique behavior of some L12-ordered intermetallics was first described by Kear and Wilsdorf[2] and later explained in detail by Copley and Kear[3] as well as Takeuchi and Kuramoto.[4,5] The Ni-base superalloys are already operating at up to about 90 pct of their melting temperature. Any further significant improvements in operating temperature require the development of a new generation of materials, with higher melting points.[6] One approach is to mimic the microstructure of the Ni-base superalloys in an analogous higher melting-point system. Possible candidates for the fcc matrix are the high-melting platinum group metals Pt, Rh, and Ir. The international state-of-the-art of these so-called refractory superalloys based on Ir and Rh is led by the group of Harada and Yamabe-Mitarai at the National Institute for Materials Science (NIMS) in Japan.[7–10] However, a major drawback of Ir and Rh alloys is their brittleness.[11] Fairbank and co-workers[12,13] (University of Cambridge, Cambridge, United Kingdom) focused their research on Pt-Zr and Pt-Hf binary alloys as well as on Pt-Rh-Zr and Pt-Rh-Hf M. HÜLLER, formerly with Metallic Materials, University Bayreuth, D-95440 Bayreuth, Germany, is Doctoral Student, Corporate Research Centre Germany, EADS Deutschland GmbH, D-81663 Munich, Germany. M. WENDEROTH, Doctoral Student, U. GLATZEL, Professor, and R. VÖLKL, Senior Researcher, are with Metallic Materials, University Bayreuth. Contact e-mail: [email protected] S. VORBERG, Doctoral Student, and B. FISCHER, Professor, are with the University of Applied Sciences Jena, FB WT, D-07745 Jena, Germany. Manuscript submitted July 2, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ternary alloys. Some of these alloys show very promising mechanical properties, but suffer from weak oxidation resistance. In a coordinated effort, research groups in South Africa[13–21] and Germany[20,21] are investigating refractory superalloys based on Pt. Hill et al.[14,15] conducted an extensive assessment of systems with Pt as the major constituent. Alloys based on Pt-Al turned out to have the highest potential, not only because of possible precipitation strengthening through Pt3Al but also due to high oxidation resistance.[16] Although stoichiometric Pt3Al with L12 structure does not show an
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